The Lake Toba ( Indonesian : Danau Toba ) is a 87 km long and 27 km wide lake in the north of the Indonesian island of Sumatra in the province of North Sumatra . The Lake Toba is a volcanic-tectonic intrusion boiler, 77 km south of the provincial capital of Medan . The level of the lake is 905 meters above sea level, the shoreline is 279 km. With a total area of 1776.5 km ² (for comparison: Constance 536 km ²), including the 640-acre island of Samosir , he is the biggest crater on earth. Prapat on the east bank is the largest and most important city on the lake. Further, the infrastructure for major cities around the lake are Berastagi in the northeast, Pematang Siantar in the east and Sibolga on the Indian Ocean.
The longitudinally extending through Sumatra Barisangebirge is a plate-tectonic uplift, caused by the pressure of the approach from the southwest are pushing Indo-oceanic plate. After the Dutch geologist Reinout Willem van Bemmelen was in the process between the Pliocene and Pleistocene in and around the present-day Lake Toba a dome-shaped, up to 2000 m high arch, which he gave the name “Batak tumor.” The extent of this bulge goes far beyond that of today’s Lake: The length along the longitudinal axis of Sumatra is 275 km, width up to 150 km. While the swelling caused by larger and larger and deeper expansion cracks, until about 74,000 years through the cracks, the gas-rich magma of the little deeper batholiths found its way into a giant eruption.
Lake Toba on the borders of dormant volcanoes Dolok Pusukbukit (west) and Dolok Tandukbenua (north-west). Lake Toba is drained by the 150 km long river Asahan, in the extreme southeast of the Bay of Porsea. Downstream breaks through the Asahan with up to 300 m deep gorge, the mountains of the eastern edge of collapse boiler. As a result of deep erosion in the level of the lake has dropped to about 250 m.